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Complete HF1 beacon-chain.md

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@ -1,4 +1,4 @@
# Ethereum 2.0 Light Client Support # Ethereum 2.0 HF1
## Table of contents ## Table of contents
@ -36,13 +36,39 @@
## Introduction ## Introduction
This is a standalone beacon chain patch adding light client support via sync committees. This is a patch implementing the first hard fork to the beacon chain, tentatively named HF1 pending a permanent name. It has three main features:
* Light client support via sync committees
* Incentive accounting reforms, reducing spec complexity and reducing the cost of processing chains that have very little or zero participation for a long span of epochs
* Fork choice rule changes to address weaknesses recently discovered in the existing fork choice
## Constants ## Constants
### Participation flags
| Name | Value | | Name | Value |
| - | - | | - | - |
| `BASE_REWARDS_PER_EPOCH` | `uint64(5)` | | `TIMELY_HEAD_FLAG` | `0` |
| `TIMELY_SOURCE_FLAG` | `1` |
| `TIMELY_TARGET_FLAG` | `2` |
### Participation rewards
| Name | Value |
| - | - |
| `TIMELY_HEAD_NUMERATOR` | `12` |
| `TIMELY_SOURCE_NUMERATOR` | `12` |
| `TIMELY_TARGET_NUMERATOR` | `32` |
| `REWARD_DENOMINATOR` | `64` |
The reward fractions add up to 7/8, leaving the remaining 1/8 for proposer rewards and other future micro-rewards.
### Misc
| Name | Value |
| - | - |
| `PARTICIPATION_FLAGS_LENGTH` | `8` |
| `FLAGS_AND_NUMERATORS` | `((TIMELY_HEAD_FLAG, TIMELY_HEAD_NUMERATOR), (TIMELY_SOURCE_FLAG, TIMELY_SOURCE_NUMERATOR), (TIMELY_TARGET_FLAG, TIMELY_TARGET_NUMERATOR))` |
## Configuration ## Configuration
@ -90,10 +116,41 @@ class BeaconBlockBody(phase0.BeaconBlockBody):
#### `BeaconState` #### `BeaconState`
```python ```python
class BeaconState(phase0.BeaconState): class BeaconState(Container):
# Sync committees # Versioning
genesis_time: uint64
genesis_validators_root: Root
slot: Slot
fork: Fork
# History
latest_block_header: BeaconBlockHeader
block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
historical_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
# Eth1
eth1_data: Eth1Data
eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
eth1_deposit_index: uint64
# Registry
validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
# Randomness
randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR]
# Slashings
slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR] # Per-epoch sums of slashed effective balances
# Participation
previous_epoch_participation: List[Bitvector[PARTICIPATION_FLAGS_LENGTH], VALIDATOR_REGISTRY_LIMIT]
current_epoch_participation: List[Bitvector[PARTICIPATION_FLAGS_LENGTH], VALIDATOR_REGISTRY_LIMIT]
# Finality
justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH] # Bit set for every recent justified epoch
previous_justified_checkpoint: Checkpoint
current_justified_checkpoint: Checkpoint
finalized_checkpoint: Checkpoint
# Light client sync committees
current_sync_committee: SyncCommittee current_sync_committee: SyncCommittee
next_sync_committee: SyncCommittee next_sync_committee: SyncCommittee
# Denominator to real-time-updated balances (NOT effective balances!)
balance_denominator: uint64
``` ```
### New containers ### New containers
@ -110,6 +167,24 @@ class SyncCommittee(Container):
### `Predicates` ### `Predicates`
#### `get_base_reward`
*Note*: The function `get_base_reward` is modified with the removal of `BASE_REWARDS_PER_EPOCH`. Additionally, it is split into `get_base_reward_per_eth` to
```python
def get_base_reward(state: BeaconState, index: ValidatorIndex) -> Gwei:
return get_base_reward_per_eth(state) * state.validators[index].effective_balance // GWEI_PER_ETH
```
##### `get_base_reward_per_eth`
```python
def get_base_reward_per_eth(state: BeaconState) -> Gwei:
total_balance = get_total_active_balance(state)
total_balance = get_total_active_balance(state)
effective_balance = state.validators[index].effective_balance
return Gwei(GWEI_PER_ETH * BASE_REWARD_FACTOR // integer_squareroot(total_balance))
#### `eth2_fast_aggregate_verify` #### `eth2_fast_aggregate_verify`
```python ```python
@ -166,6 +241,69 @@ def get_sync_committee(state: BeaconState, epoch: Epoch) -> SyncCommittee:
return SyncCommittee(pubkeys=pubkeys, pubkey_aggregates=aggregates) return SyncCommittee(pubkeys=pubkeys, pubkey_aggregates=aggregates)
``` ```
#### `get_unslashed_participating_indices`
```python
def get_unslashed_participating_indices(state: BeaconState, flag: uint8, epoch: Epoch) -> Set[ValidatorIndex]:
assert epoch in (get_previous_epoch(state), get_current_epoch(state))
if epoch == get_current_epoch(state):
epoch_participation = state.current_epoch_participation
else:
epoch_participation = state.previous_epoch_participation
participating_indices = [index in get_active_validator_indices(state, epoch) if epoch_participation[index][flag]]
return set(filter(lambda index: not state.validators[index].slashed, participating_indices))
```
#### `get_flag_deltas`
```python
def get_flag_deltas(state: BeaconState, flag: uint8, numerator: uint64) -> Tuple[Sequence[Gwei], Gwei]:
"""
Computes the rewards and penalties associated with a particular duty, by scanning through the participation
flags to determine who participated and who did not and assigning them the appropriate rewards and penalties.
"""
rewards = [Gwei(0)] * len(state.validators)
penalties = [Gwei(0)] * len(state.validators)
unslashed_participating_indices = get_unslashed_participating_indices(state, flag, get_previous_epoch(state))
increment = EFFECTIVE_BALANCE_INCREMENT # Factored out from balances to avoid uint64 overflow
unslashed_participating_increments = get_total_balance(state, unslashed_participating_indices) // increment
active_increments = get_total_active_balance(state) // increment
for index in get_eligible_validator_indices(state):
base_reward = get_base_reward(state, index)
if index in unslashed_participating_indices:
if is_in_inactivity_leak(state):
# Optimal participatition is fully rewarded to cancel the inactivity penalty
rewards[index] = base_reward * numerator // REWARD_DENOMINATOR
else:
rewards[index] = (
(base_reward * unslashed_participating_increments // active_increments + base_reward)
* numerator // REWARD_DENOMINATOR
)
return rewards, get_base_reward_per_eth(state) * numerator // REWARD_DENOMINATOR
```
##### `get_inactivity_penalty_deltas`
*Note*: The function `get_inactivity_penalty_deltas` is modified in the selection of matching target indices and the removal of `BASE_REWARDS_PER_EPOCH`.
```python
def get_inactivity_penalty_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
"""
Compute the penalties associated with the inactivity leak, by scanning through the participation
flags to determine who participated and who did not, applying the leak penalty globally and applying
compensatory rewards to participants.
"""
rewards = [Gwei(0) for _ in range(len(state.validators))]
if is_in_inactivity_leak(state):
reward_numerator_sum = sum(numerator for (_, numerator) in FLAGS_AND_NUMERATORS)
matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG, get_previous_epoch(state))
for index in get_eligible_validator_indices(state):
if index in matching_target_attesting_indices:
effective_balance = state.validators[index].effective_balance
rewards[index] += Gwei(effective_balance * get_finality_delay(state) // INACTIVITY_PENALTY_QUOTIENT)
return rewards, Gwei(GWEI_PER_ETH * get_finality_delay(state) // INACTIVITY_PENALTY_QUOTIENT)
```
### Block processing ### Block processing
```python ```python
@ -178,6 +316,52 @@ def process_block(state: BeaconState, block: BeaconBlock) -> None:
process_sync_committee(state, block.body) process_sync_committee(state, block.body)
``` ```
#### New `process_attestation`
*Note*: The function `process_attestation` is modified to do incentive accounting with epoch participation flags.
```python
def process_attestation(state: BeaconState, attestation: Attestation) -> None:
data = attestation.data
assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state))
assert data.target.epoch == compute_epoch_at_slot(data.slot)
assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= data.slot + SLOTS_PER_EPOCH
assert data.index < get_committee_count_per_slot(state, data.target.epoch)
committee = get_beacon_committee(state, data.slot, data.index)
assert len(attestation.aggregation_bits) == len(committee)
if data.target.epoch == get_current_epoch(state):
epoch_participation = state.current_epoch_participation
justified_checkpoint = state.current_justified_checkpoint
else:
epoch_participation = state.previous_epoch_participation
justified_checkpoint = state.previous_justified_checkpoint
# Matching roots
is_matching_head = data.beacon_block_root == get_block_root_at_slot(state, data.slot)
is_matching_source = data.source == justified_checkpoint
is_matching_target = data.target.root == get_block_root(state, data.target.epoch)
assert is_matching_source
# Participation flags
participation_flags = []
if is_matching_head and state.slot <= data.slot + MIN_ATTESTATION_INCLUSION_DELAY:
participation_flags.append(TIMELY_HEAD_FLAG)
if is_matching_source and state.slot <= data.slot + integer_squareroot(SLOTS_PER_EPOCH):
participation_flags.append(TIMELY_SOURCE_FLAG)
if is_matching_target and state.slot <= data.slot + SLOTS_PER_EPOCH:
participation_flags.append(TIMELY_TARGET_FLAG)
# Update epoch participation flags
proposer_reward_numerator = 0
for index in get_attesting_indices(state, data, attestation.aggregation_bits):
for flag, numerator in FLAGS_AND_NUMERATORS:
if flag in participation_flags and not epoch_participation[index][flag]:
epoch_participation[index][flag] = True
proposer_reward_numerator += get_base_reward(state, index) * numerator
# Reward proposer
proposer_reward = Gwei(proposer_reward_numerator // (REWARD_DENOMINATOR * PROPOSER_REWARD_QUOTIENT))
increase_balance(state, get_beacon_proposer_index(state), proposer_reward)
# Verify signature
assert is_valid_indexed_attestation(state, get_indexed_attestation(state, attestation))
```
#### Sync committee processing #### Sync committee processing
```python ```python
@ -208,42 +392,108 @@ def process_sync_committee(state: BeaconState, body: BeaconBlockBody) -> None:
### Epoch processing ### Epoch processing
#### Components of attestation deltas #### New `process_justification_and_finalization`
*Note*: The function `get_inactivity_penalty_deltas` is modified with `BASE_REWARDS_PER_EPOCH` replaced by `BASE_REWARDS_PER_EPOCH - 1`. *Note*: The function `process_justification_and_finalization` is modified with `matching_target_attestations` replaced by `matching_target_indices`.
```python ```python
def get_inactivity_penalty_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]: def process_justification_and_finalization(state: BeaconState) -> None:
""" # Initial FFG checkpoint values have a `0x00` stub for `root`.
Return inactivity reward/penalty deltas for each validator. # Skip FFG updates in the first two epochs to avoid corner cases that might result in modifying this stub.
""" if get_current_epoch(state) <= GENESIS_EPOCH + 1:
penalties = [Gwei(0) for _ in range(len(state.validators))] return
if is_in_inactivity_leak(state): previous_epoch = get_previous_epoch(state)
matching_target_attestations = get_matching_target_attestations(state, get_previous_epoch(state)) current_epoch = get_current_epoch(state)
matching_target_attesting_indices = get_unslashed_attesting_indices(state, matching_target_attestations) old_previous_justified_checkpoint = state.previous_justified_checkpoint
for index in get_eligible_validator_indices(state): old_current_justified_checkpoint = state.current_justified_checkpoint
# Penalize validator so that optimal attestation performance is rewarded with one base reward per epoch # Process justifications
base_reward = get_base_reward(state, index) state.previous_justified_checkpoint = state.current_justified_checkpoint
penalties[index] += Gwei((BASE_REWARDS_PER_EPOCH - 1) * base_reward - get_proposer_reward(state, index)) state.justification_bits[1:] = state.justification_bits[:JUSTIFICATION_BITS_LENGTH - 1]
if index not in matching_target_attesting_indices: state.justification_bits[0] = 0b0
effective_balance = state.validators[index].effective_balance matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG, previous_epoch)
penalties[index] += Gwei(effective_balance * get_finality_delay(state) // INACTIVITY_PENALTY_QUOTIENT) if get_total_balance(state, matching_target_indices) * 3 >= get_total_active_balance(state) * 2:
state.current_justified_checkpoint = Checkpoint(epoch=previous_epoch,
root=get_block_root(state, previous_epoch))
state.justification_bits[1] = 0b1
matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG, current_epoch)
if get_total_balance(state, matching_target_indices) * 3 >= get_total_active_balance(state) * 2:
state.current_justified_checkpoint = Checkpoint(epoch=current_epoch,
root=get_block_root(state, current_epoch))
state.justification_bits[0] = 0b1
# Process finalizations
bits = state.justification_bits
# The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th as source
if all(bits[1:4]) and old_previous_justified_checkpoint.epoch + 3 == current_epoch:
state.finalized_checkpoint = old_previous_justified_checkpoint
# The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as source
if all(bits[1:3]) and old_previous_justified_checkpoint.epoch + 2 == current_epoch:
state.finalized_checkpoint = old_previous_justified_checkpoint
# The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as source
if all(bits[0:3]) and old_current_justified_checkpoint.epoch + 2 == current_epoch:
state.finalized_checkpoint = old_current_justified_checkpoint
# The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source
if all(bits[0:2]) and old_current_justified_checkpoint.epoch + 1 == current_epoch:
state.finalized_checkpoint = old_current_justified_checkpoint
```
# No rewards associated with inactivity penalties #### New `process_rewards_and_penalties`
rewards = [Gwei(0) for _ in range(len(state.validators))]
return rewards, penalties *Note*: The function `process_rewards_and_penalties` is modified to use participation flag deltas.
```python
def process_rewards_and_penalties(state: BeaconState) -> None:
# No rewards are applied at the end of `GENESIS_EPOCH` because rewards are for work done in the previous epoch
if get_current_epoch(state) == GENESIS_EPOCH:
return
flag_deltas = [get_flag_deltas(state, flag, numerator) for (flag, numerator) in FLAGS_AND_NUMERATORS]
deltas = flag_deltas + [get_inactivity_penalty_deltas(state)]
for (rewards, penalty) in deltas:
for index in range(len(state.validators)):
increase_balance(state, ValidatorIndex(index), rewards[index])
# Bounds-friendly expansion for `denom *= (1 + penalty // GWEI_PER_ETH)`
state.balance_denominator = (
state.balance_denominator +
penalty +
(state.balance_denominator - GWEI_PER_ETH) * penalty // GWEI_PER_ETH
)
``` ```
#### Final updates #### Final updates
*Note*: The function `process_final_updates` is modified to handle sync committee updates. *Note*: The function `process_final_updates` is modified to handle sync committee updates, replacement of `PendingAttestation`s with participation flags, and a sliding-denominator-aware hysteresis mechanism.
```python ```python
def process_final_updates(state: BeaconState) -> None: def process_final_updates(state: BeaconState) -> None:
# FIXME: unfold the full `process_final_updates` to avoid side effects. current_epoch = get_current_epoch(state)
phase0.process_final_updates(state) next_epoch = Epoch(current_epoch + 1)
next_epoch = get_current_epoch(state) + Epoch(1) # Reset eth1 data votes
if next_epoch % EPOCHS_PER_ETH1_VOTING_PERIOD == 0:
state.eth1_data_votes = []
# Update sync committees
if next_epoch % EPOCHS_PER_SYNC_COMMITTEE_PERIOD == 0: if next_epoch % EPOCHS_PER_SYNC_COMMITTEE_PERIOD == 0:
state.current_sync_committee = state.next_sync_committee state.current_sync_committee = state.next_sync_committee
state.next_sync_committee = get_sync_committee(state, next_epoch + EPOCHS_PER_SYNC_COMMITTEE_PERIOD) state.next_sync_committee = get_sync_committee(state, next_epoch + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
``` # Update effective balances with hysteresis
for index, validator in enumerate(state.validators):
balance_increments = state.balances[index] * HYSTERESIS_QUOTIENT // state.balance_denominator
effective_increments = validator.effective_balance // GWEI_PER_ETH * HYSTERESIS_QUOTIENT
if (
balance_increments + HYSTERESIS_DOWNWARD_MULTIPLIER < effective_increments
or effective_increments + HYSTERESIS_UPWARD_MULTIPLIER < balance_increments
or get_current_epoch(state) == validator.withdrawable_epoch
):
validator.effective_balance = (state.balances[index] // state.balance_denominator) * GWEI_PER_ETH
if state.balance_denominator >= 2 * GWEI_PER_ETH:
state.balances = [x//2 for x in state.balances]
state.balance_denominator //= 2
# Reset slashings
state.slashings[next_epoch % EPOCHS_PER_SLASHINGS_VECTOR] = Gwei(0)
# Set randao mix
state.randao_mixes[next_epoch % EPOCHS_PER_HISTORICAL_VECTOR] = get_randao_mix(state, current_epoch)
# Set historical root accumulator
if next_epoch % (SLOTS_PER_HISTORICAL_ROOT // SLOTS_PER_EPOCH) == 0:
historical_batch = HistoricalBatch(block_roots=state.block_roots, state_roots=state.state_roots)
state.historical_roots.append(hash_tree_root(historical_batch))
# Rotate current/previous epoch participation flags
state.previous_epoch_participation = state.current_epoch_participation
state.current_epoch_participation = []